1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
From a viewpoint of cost reduction of an optical scanning device, a scanning lens, which is one of optical elements that make up the optical scanning device, forms a large proportion of the cost. Cost of the scanning lens chiefly depends on length of time necessary for molding the scanning lens. Put another way, the shorter the molding time, the less the cost. Meanwhile, molding of a scanning lens is typically performed by injecting heated and softened resin into a molding die and fill the die with the resin. During this molding, refractive index gradient that depends on the molding time is produced in the scanning lens. Such refractive index gradient in the scanning lens generally degrades optical performance. Causes of the degradation in optical performance include scan line bow.
FIG. 1 is a explanatory schematic diagram of “refractive index gradient in a lens” of a scanning lens 234 (see FIG. 5). FIG. 1(a) illustrates refractive index gradient in a main-scanning cross section of the scanning lens 234 “in a contour map form”. The refractive index changes along a dash-dot line of FIG. 1(b) in such a manner that the refractive index gradually increases from a center portion of the lens toward both end portions (in the main-scanning direction) of the lens as illustrated in FIG. 1(b). FIG. 1(c) illustrates refractive index gradient in a sub-scanning cross section (planar cross section containing the optical axis of the scanning lens 234 and parallel to the sub-scanning direction) of the scanning lens 234 in a contour map form. FIG. 1(d) illustrates how the refractive index on the optical axis changes in the sub-scanning cross section. FIG. 1(e) illustrates the refractive index gradient in the sub-scanning direction in the sub-scanning cross section.
As illustrated in FIG. 1(e), the refractive index gradient in the sub-scanning direction is graded in such a manner that “the refractive index increases as the distance from the optical axis in the sub-scanning direction increases”. This tendency of the refractive index gradient in the sub-scanning direction that “the refractive index increases as the distance from the optical axis increases in the sub-scanning direction” is not specific to the sub-scanning cross section of the scanning lens 234 but common to any planar cross section parallel to the sub-scanning cross section.
However, “the greater the distance in the main-scanning direction” from the optical axis of the scanning lens 234, the smaller the refractive index change (i.e., a difference between a refractive index in the main-scanning cross section and a refractive index of edge portions in the sub-scanning direction) in the refractive index gradient in the sub-scanning direction becomes. This is readily understood from FIG. 1(a), in which the greater the distance from the optical axis, the greater “the intervals between contour lines of the refractive index” and the change in the refractive index becomes smaller.
The scan line bow, which is a problem to be solved by of the present invention, is chiefly affected by the refractive index gradient in the sub-scanning direction (see FIG. 1(e)) among the refractive index gradients described above. Effect of the refractive index gradient in the sub-scanning direction on the optical performance scanning lens 234 manifests itself as “variation of imaging focal length in the sub-scanning direction”.
Here, it is assume that the scanning lens 234 has such refractive index gradient as that illustrated in FIG. 2. Illustrated in FIG. 2 is an example for reference purpose where the refractive index is uniform. FIG. 2 is a diagram indicating dependence of differential of refractive index change (first-order derivative of the refractive index change) in the sub-scanning direction on lens height. The lens height is a position in the lens in the main-scanning direction. The lens height is zero on the optical axis in the main-scanning direction.
When the refractive index is uniform, the differential of the refractive index change is constant. In this case, a scan line bow that is 25 μm maximum occurs as illustrated in FIG. 3. This scan line bow leads to degradation of quality of output images of the optical scanning device.
Measures for correcting scan line bow that occurs in what is referred to as an oblique incidence optical system have already been proposed (see Japanese Patent Application Laid-open No. 2007-025536, for example).
Therefore, it is desirable to provide an optical scanning device capable of preventing degradation in quality in output images even when the optical scanning device includes a scanning lens that has refractive index gradient, and an image forming apparatus including the optical scanning device.